Synthesis and bonding analysis of pentagonal bipyramidal rhenium carboxamide oxo complexes

Seven-coordinate rhenium oxo complexes supported by a tetradentate bipyridine carboxamide/carboxamidate ligand are reported. The neutral dicarboxamide H₂^Phbpy-da ligand initially coordinates in an L₄ (ONNO) fashion to an octahedral rhenium oxo precursor, yielding a seven-coordinate rhenium oxo complex. Subsequent deprotonation generates a new oxo complex featuring the dianionic (L₂X₂) carboxamidate (NNNN) form of the ligand. Computational studies provide insight into the relative stability of possible linkage isomers upon deprotonation. Structural studies and molecular orbital theory are employed to rationalize the relative isomer stability and provide insight into the rhenium-oxo bond order.